Fonts in GNOME Flatpak apps are not anti-aliased on non-GTK desktops

Under Wayland GNOME apps get their anti-aliasing settings from XDG Portals. To make fonts look nice in GNOME apps on Wayland and a non-GTK desktop (e.g. KDE Plasma) you need the appropriate portal installed: 
 pacman -S xdg-desktop-portal-gtk
 
 Then, restart the xdg-desktop-portal and xdg-desktop-portal-gtk user unit: 
 systemctl --user restart xdg-desktop-portal xdg-desktop-portal-gtk
 
 After that restart your GNOME flatpak app and fonts should now be anti-aliased.